Power converter

ABSTRACT

A power converter that can continue to operate after suffering a partial failure. The power converter includes multiple array transformers; normally-on switches connected respectively in series with the ends of each of the primary windings of the array transformers; normally-off current bypass devices connected in parallel with the series connections of the primary windings of the array transformers and the switches at the transformer ends; AC-DC converter units having AC sides respectively connected secondary windings of the array transformers; and mutually independent DC circuits respectively connected to DC sides of the AC-DC converter units. By turning on the current bypass device of the primary winding of a specified array transformer and turning off the switches at the ends of that primary winding it is possible to isolate the specified array transformer and the AC-DC converter unit connected to it.

TECHNICAL FIELD

This invention relates to a power converter connected in series with aline such as a power line, a distribution line or a single-phase ACwire, and particularly to a power converter capable of continuing tooperate after suffering a partial failure.

BACKGROUND ART

FIG. 8 is a circuit diagram showing the construction of a powerconverter of related art disclosed in for example U.S. Pat. No.5,646,511. In this power converter, a series transformer having itsprimary winding connected in series with a power line, a distributionline or a single-phase AC wire for an electric train or the like(hereinafter abbreviated to ‘line’) and multiple array transformers onthe secondary side of this series transformer are combined in two stagesto connect AC-DC converter units to the line. This power converter hasthe function of a line power tide current control apparatus. In thefigure, the primary winding 201 of a series transformer 200 is connectedbetween the power supply side 1 of a line and a power supply side orload side 2 of the line. The primary windings 411 to 441 of arraytransformers 410 to 440 (the case of a four-stage array is shown) areconnected in series with the secondary winding 202 of the seriestransformer 200. The AC sides of AC-DC converter units 510 to 540 arerespectively connected to the secondary windings 412 to 442 of the arraytransformers 410 to 440, and the DC sides of the four AC-DC converterunits 510 to 540 are connected to a common DC circuit 511.

Because power converters of related art have been constructed like this,if even one of the multiple AC-DC converter units 510 to 540 fails,because it is impossible to maintain the DC voltage of the DC circuit511, none of the AC-DC converter units can be operated, and it has beennecessary to shut down the power converter. And there has been theproblem that the power converter has to be shut down until repair orperiodic checking is complete, and the availability of the system falls.

The present invention was made to solve these problems, and it is anobject of the invention to provide a power converter capable ofcontinuing to operate as a system even when one of multiple AC-DCconverter units fails or is stopped for a periodic check.

DISCLOSURE OF THE INVENTION

The invention provides a power converter having a series transformerwith its primary winding connected in series with a line, multiple arraytransformers connected in series with the secondary winding of thisseries transformer, normally-on switches connected in series with theends of the primary windings of the array transformers, normally-offcurrent bypass devices connected in parallel with the series connectionsof the primary windings of the array transformers and the switches attheir ends, AC-DC converter units having their AC sides connected toeach of the secondary windings of the multiple array transformers, andmutually independent DC circuits severally connected to the DC sides ofthe AC-DC converter units, wherein by turning on the current bypassdevice of the primary winding of a specified array transformer andturning off the switches at the ends of that primary winding it ispossible to isolate the specified array transformer and the AC-DCconverter unit connected to it.

By this means it is possible not only to increase the availability andthe reliability of the apparatus as a whole but also to raise thecapacity of the apparatus by easily making additions to it.

The invention also provides a power converter including multiple arraytransformers having their primary windings connected to a line inseries, normally-on switches connected in series with the ends of theprimary windings of the array transformers, normally-off first currentbypass devices connected in parallel with the series connections of theprimary windings of the array transformers and the switches connected totheir ends, AC-DC converter units having their AC sides connected to thesecondary windings of each of the array transformers, mutuallyindependent DC circuits connected to the DC sides of the AC-DC converterunits, and a normally-off second current bypass device connected inparallel with all of the series-connected array transformers, wherein byturning on the first current bypass device of the primary winding of aspecified array transformer and turning off the switches at the ends ofthat primary winding it is possible to isolate the specified arraytransformer and the AC-DC converter unit connected to it.

By this means it is possible not only to increase the availability andthe reliability of the apparatus as a whole but also to raise thecapacity of the apparatus by easily making additions to it.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the construction of a firstpreferred embodiment of a power converter according to the invention;

FIG. 2 is a circuit diagram showing the construction of a powerconverter of a second preferred embodiment;

FIG. 3 is a circuit diagram showing the construction of an ordinarysingle-phase AC-DC converter;

FIG. 4 is a circuit diagram showing the construction of a powerconverter of a third preferred embodiment;

FIG. 5 is a circuit diagram showing the construction of a powerconverter of a fourth preferred embodiment;

FIG. 6 is a circuit diagram showing the construction of a powerconverter of a fifth preferred embodiment;

FIG. 7 is a circuit diagram showing the construction of a powerconverter of a sixth preferred embodiment; and

FIG. 8 is a circuit diagram showing the construction of a powerconverter of related art.

BEST MODE FOR CARRYING OUT THE INVENTION

First Preferred Embodiment

FIG. 1 is a circuit diagram showing the construction of a firstpreferred embodiment of a power converter according to the invention.(Throughout the figures, the same reference numerals denote the same orequivalent parts and hereinafter referred to as the same meaning.) InFIG. 1, the primary winding 201 of a series transformer 200 is connectedin series between a power supply side 1 of a line and a power supplyside or load side 2 of the line. The primary windings 411 to 441 ofarray transformers 410 to 440 (the case of a four-stage array is shown)are connected in series to the secondary winding 202 of the seriestransformer 200. The AC sides of four AC-DC converter units 510 to 540are respectively connected to the secondary windings 412 to 442 of thearray transformers 410 to 440, and mutually independent DC circuits 511to 541 are connected the DC sides of the AC-DC converter units 510 to540.

This power converter according to the invention is of a constructionsuch that the individual DC circuits 511 to 541 of the AC-DC converterunits 510 to 540 are made mutually independent, and are not connected tothe DC circuit of any other AC-DC converter unit. Normally-on switches(circuit breakers, isolators or semiconductor switches) 311 to 341 and312 to 342 are disposed (connected) in series with the ends of theprimary windings 411 to 441 of the array transformers 410 to 440, andnormally-off current bypass devices (circuit breakers, isolators orsemiconductor switches) 310 to 340 are disposed (connected) in parallelwith the series connections of the primary windings 411 to 441 of eachof the array transformers 410 to 440 and the switches 311 to 341 and 312to 342 at their ends. Consequently, the construction is such that whenthe current bypass device of the primary winding of a certain arraytransformer is turned on and the switches at the ends of that primarywinding are turned off, that array transformer is cut off from the line.

Also, switches (circuit breakers, isolators or semiconductor switches)101 and 102 are disposed (connected) in series with the ends of theprimary winding 201 of the series transformer 200, and a current bypassdevice (circuit breaker, isolator or semiconductor switch) 103 isdisposed (connected) in parallel with the series connection of theprimary winding 201 of the series transformer 200 and the two switches101, 102.

Further, a short-circuiting switch (circuit breaker, isolator orsemiconductor switch) 300 for short-circuiting the secondary winding 202of the series transformer 200 is provided, so that the AC-DC converterunits 510 to 540 and the array transformers 410 to 440 can all beprotected from short-circuit current at the time of a line accident. Insome cases, because the current bypass devices 310 to 340 are present,the short-circuiting device 300 may be dispensed with. This decision canbe made on the basis of the design considerations of cost, availablespace, and redundancy.

In this first preferred embodiment, as the winding arrangement of thesecondary winding 202 of the series transformer 200, delta connection, Yconnection, or single-phase connection can be used. And also as thewinding arrangement of each of the secondary windings 412 to 442 of thearray transformers 410 to 440, delta connection, Y connection, orsingle-phase connection can be used.

Next, the operation of this power converter will be explained. Acharacterizing feature of a power converter connected in series with aline is that the AC-DC converter units 510 to 540 themselves cannotdirectly control the currents flowing to the units, and what theycontrol is only the sizes and the phases of the voltages that the AC-DCconverter units 510 to 540 output. The reason that the power convertercan control the current of the line indirectly is that the vector sum ofthe output voltages of the AC-DC converter units 510 to 540 produces avoltage in the primary winding 201 of the series transformer 200 by wayof the array transformers 410 to 440, and by that injection voltageproducing a voltage of a certain phase and a certain size between line 1and line 2, with all the voltage sources and current sources on the linenetwork, and all the line impedances, it is possible to change thecurrent passing through the power converter. In this sense, this powerconverter has the function of a line power tide current controlapparatus. Because of this, as the AC-DC converter units 510 to 540 ofthe power converter, voltage sourced converters, which constitutevoltage sources, are employed. As a consequence of this operatingprinciple, it is not necessary for all the AC-DC converter units toproduce the same voltage, and even if one AC-DC converter unit hasstopped, the power converter can operate without any problem.

During normal operation of the power converter of the first preferredembodiment, the current bypass device 103 is in its OFF state, theswitches 101 and 102 are in their ON states, the short-circuiting device300 is in its OFF state, the current bypass devices 310 to 340 are intheir OFF states, and the switches 311 to 341 and 312 to 342 are intheir ON states.

It will now be supposed that the AC-DC converter unit 510 has paralleledout from the line due to a failure. At this time, because the currentbypass device 310 has been turned on and the switches 311 and 312 havebeen turned off, and, in this first preferred embodiment, because the DCcircuit 511 has been electrically cut off from the DC circuits 521 to541 of the other AC-DC converter units, the power converter can continueto operate. The reason why it has not been possible for a powerconverter of related art to operate with one or more AC-DC converterunits paralleled out is that the AC-DC converter units have not beenindependent on either the AC side or the DC side.

The group of AC-DC converter units 510 to 540 as a whole is required toproduce a specified differential voltage in the primary winding 201 ofthe series transformer 200. In this first preferred embodiment, becausethe AC-DC converter units 510 to 540 are constructed independently, evenif one or more of these AC-DC converter units parallels out, the powerconverter can still operate.

When the required specifications of the power converter are satisfied bya number of AC-DC converter units (N), if redundancy of one or more (n)units is added and (N+n) AC-DC converter units are provided, then evenif n AC-DC converter units fail, operation is possible without themaximum rating of the system being lost. Consequently, if AC-DCconverter units are provided as redundancy, operation at the 100% ratingof the power converter is possible even with the number of AC-DCconverter units corresponding to the redundancy paralleled out. By thismeans it is possible to obtain a highly reliable installation.

When the maximum currents flowing through the AC-DC converter units 510to 540 at the time of a line accident are greater than the ratedcurrents of the AC-DC converter units 510 to 540, by the number ofstages of array transformers and AC-DC converter units in theconstruction being increased, the maximum currents of the AC-DCconverter units 510 to 540 can be reduced. This results from the natureof a power converter connected to a line in series. If the rating of thepower converter is defined as the product of the voltage Vs injectedinto the primary winding of the series transformer 200 and the maximumcurrent Is of the line, then the number of stages (N) can be obtained bydividing the rating of the power converter (Vs×Is) by the product of therated voltage Vc and the rated current Ic of the array transformer andAC-DC converter unit of one stage (Vc×Ic). Because with just the numberof stages N1 obtained from the rated current of the line at normal timesthe maximum current of the AC-DC converter units will exceed the ratedcurrent at the time of an accident or the like, it is desirable to takeinto account a maximum current Is2 as of the time of a line accident insetting a rated current Ic2 of the AC-DC converter units, and to use theAC-DC converter units and the array transformers derated so that theyare normally used below their maximum ratings. Because this meansdesigning to lower the voltage of the primary windings 411 to 441 of thearray transformers 410 to 440, as a result the number of stages Nincreases.

Also, in this first preferred embodiment, utilizing the nature of thepower converter connected in series with a line shown above, it ispossible to increase the capacity of the power converter by increasingthe number of array transformers and AC-DC converter units in series,even after the power converter is installed. This characteristic featureis possible because the DC circuits of the AC-DC converter units areindependent.

And, in this first preferred embodiment, in the series transformer 200,normally, when the voltage of the secondary winding has fallen, themaximum current of the secondary winding 202 increases at the time of aline accident. When the maximum current of the secondary winding 202exceeds the rated current of the semiconductor switch (short-circuitingdevice) 300 or the circuit breakers (current bypass devices) 310 to 340or the isolators (switches) 311 to 341 and 312 to 342, in the seriestransformer 200, reversely, in the first preferred embodiment it is alsopossible to employ a method of optimizing the ratings of thesemiconductor switch 300, the circuit breakers 310 to 340 and theisolators 311 to 341 and 312 to 342 by increasing the voltage of thesecondary winding.

Second Preferred Embodiment

Whereas in the first preferred embodiment a construction was adoptedsuch that the AC-DC converter units 510 to 540 could be isolated one ata time, as shown in FIG. 2, AC-DC converter units 550 to 580 areconnected to the secondary windings 452 and 462 of array transformers450 and 460 two by two. In this case, although the two DC circuits 551and 552 forming one pair are common, they are independent from the DCcircuits 561 and 562 forming another pair. This point is similar to thefirst preferred embodiment.

When the number of AC-DC converter units is 2×N, compared to the firstpreferred embodiment, because the number of array transformer stages isN, i.e. half the number in the first preferred embodiment, themanufacturing cost of the array transformers can be expected to becheaper. In this second preferred embodiment the AC-DC converter unitshave to be shut down in twos when there is a failure of an AC-DCconverter unit or for a periodic check, but if redundancy is notaffected to the expected availability, an unproblematic system can beprovided. And, in this second preferred embodiment, because two AC-DCconverter units are controlled at the same time, certain circuits ofcontrol units (not shown) such as those for DC voltage control can bemade common and cut down to one circuit per two AC-DC converter units,whereby cost reductions can be achieved.

FIG. 3 is a circuit diagram showing the construction of an ordinarysingle-phase AC-DC converter (single-phase invertor). In the figure,self-quenching devices 911, 912 and flywheel diodes 921, 922 areconnected to an AC-side terminal 901, and self-quenching devices 913,914 and flywheel diodes 923, 924 are connected to an AC-side terminal902. A condenser 930 is connected to DC-side terminals. The secondpreferred embodiment can also be applied in a case where an AC-DCconverter unit constitutes a single-phase bridge of the kind shown inFIG. 3. In the case of a line such as a single-phase AC wire for anelectric train, because a 3-phase bridge AC-DC converter unit cannot beused, it is necessary to employ a single-phase bridge AC-DC converterunit of the kind shown in FIG. 3.

Third Preferred Embodiment

Whereas in the first preferred embodiment a series transformer 200 wasdisposed between a power supply side 1 of a line and a power supply sideor load side 2 of the line and array transformers 410 to 440 and AC-DCconverter units 510 to 540 were provided, the array transformers 410 to440 can also be connected in series between a power supply side 1 of aline and a power supply side or load side 2 of the line directly, as ina third preferred embodiment shown in FIG. 4. A current bypass device(circuit breaker, isolator or semiconductor switch) 300 is connected inparallel with all the primary windings 411 to 441 of the arraytransformers 410 to 440, and at the time of a line accident bypasses allof the primary windings 411 to 441 of the array transformers 410 to 440together.

The circuit in FIG. 4 can be applied in cases such as when the arraytransformers can be connected to the line directly, when semiconductorswitches can be connected to the line directly, and when the arraytransformers can transform (normally, reduce) the line voltage, likelyto be a relatively high voltage, to the AC voltage of the AC-DCconverter units in one stage.

In the first preferred embodiment, normally, when the voltage of thesecondary winding of the series transformer 200 is reduced, the currentof the secondary winding 202 increases. When consequently at the time ofa line accident the maximum current flowing through the secondarywinding 202 will become too large, a semiconductor switch 300 with alarge rated current is manufactured. When it is easier to manufacture ahigh-voltage semiconductor switch 300 with a low rated current than alow-voltage semiconductor switch 300 with a high rated current, theseries transformer 200 can be dispensed with and a construction likethat of the third preferred embodiment employed.

Fourth Preferred Embodiment

Whereas in the second preferred embodiment the series transformer 200was disposed between a power supply side 1 of a line and a power supplyside or load side 2 of the line and array transformers 450 to 460 andAC-DC converter units 550 to 580 were provided, a construction in whichthe array transformers 450 to 460 are connected in series between apower supply side 1 of a line and a power supply side or load side 2 ofthe line directly is also possible, as in the fourth embodiment shown inFIG. 5. A current bypass switch (circuit breaker, isolator orsemiconductor switch) 300 is connected in parallel with all the primarywindings 451 to 461 of the array transformers 450 to 460, and at thetime of a line accident bypasses all of the primary windings 451 to 461of the array transformers 450 to 460 together.

The circuit of FIG. 5 can be applied in cases such as when the arraytransformers can be connected to the line directly, when semiconductorswitches can be connected to the line directly, and when the arraytransformers can transform (normally, reduce) the line voltage, likelyto be a relatively high voltage, to the AC voltage of the AC-DCconverter units in one stage.

In the second preferred embodiment, normally, when the voltage of thesecondary winding of the series transformer 200 is reduced, the currentof the secondary winding 202 increases. When consequently at the time ofa line accident the maximum current flowing through the secondarywinding 202 will become too large, a semiconductor switch 300 with alarge rated current is manufactured. When it is easier to manufacture ahigh-voltage semiconductor switch 300 with a low rated current than alow-voltage semiconductor switch 300 with a high rated current, theseries transformer 200 can be dispensed with and a construction likethat of the fourth preferred embodiment employed.

Fifth Preferred Embodiment

Whereas in the first preferred embodiment one set of switches 311 to 341and 312 to 342 and current bypass devices 310 to 340 for isolating fromthe system the primary windings 411 to 441 of the array transformers 410to 440 was provided per array transformer, in a fifth preferredembodiment shown in FIG. 6, an array transformer 801 is made up oftransformers 810 and 820 connected in series. Similarly, an arraytransformer 802 is made up of a plurality of transformers 830 and 840connected in series. Normally-on switches 311 and 322 are connected inseries with the ends of the series connection of the primary windings811 and 821 of the plurality of transformers 810 and 820. Similarly,normally-on switches 331 and 342 are connected in series with the endsof the series connections of the primary windings 831 and 841 of theplurality of transformers 830 and 840. One normally-off current bypassdevice 310 is connected in parallel with the series connection of theplurality of transformers 810 and 820 and the switches 311 and 322 atthe ends. Similarly, one normally-off current bypass device 330 isconnected in parallel with the series connection of the plurality oftransformers 830 and 840 and the switches 331 and 342 at the ends. Also,AC-DC converter units 510 to 540 are respectively connected to thesecondary windings 812, 822, 832, 842 of the transformers 810 to 840. Bymeans of the construction shown in FIG. 6 it is possible to reduce cost.

This results in a loss of redundancy of the array transformers and AC-DCconverter units, but if redundancy is not a problem for the powerconverter it is a construction that can be employed.

The alteration of construction applied to the first preferred embodimentto the fifth preferred embodiment can also be applied to the second,third and fourth preferred embodiments.

Sixth Preferred Embodiment

In the first preferred embodiment, by connecting not only a capacitorbut some other energy-storing device to the DC circuit of each AC-DCconverter unit, it becomes possible for the active voltage component andthe reactive voltage component of the injection voltage outputted by thepower converter to be outputted with any phase through 360°.

Examples of energy-storing devices are secondary cells such asbatteries, energy-storing devices such as large-capacity capacitors, oranother AC-DC converter unit connected by way of a motor-cum-generatorto a mechanical energy source such as a flywheel.

A sixth preferred embodiment shown in FIG. 7 is a construction known asa DVR (Dynamic Voltage Restorer) or UPFC (Unified Power FlowController). An independent energy source can be obtained byindependently connecting the DC circuits 511 to 541 of AC-DC converterunits 510 to 540 to other AC-DC converter units 513 to 543 andconnecting these AC-DC converter units 513 to 543 individually to theline via transformers 610 to 640, circuit breakers 611 to 641, atransformer 700 and a circuit breaker 701.

INDUSTRIAL APPLICABILITY

As will be clear from the foregoing description, a power converteraccording to the invention is suitable for use in a line power tidecontrol apparatus capable of continuing to operate after suffering apartial failure.

1. A power converters comprising: a series transformer having a primarywinding connected in series with a line and having a secondary winding;multiple array transformers having respective primary and secondarywindings, the primary windings being connected in series and to thesecondary winding of the series transformer; normally-on switchesrespectively connected in series with corresponding ends of each of theprimary windings of the array transformers; normally-off current bypassdevices respectively connected in parallel with the corresponding seriesconnections of each of the primary windings of the array transformersand the switches connected to the ends of the primary winding of thecorresponding array transformer; AC-DC converter units, each AC-DCconverter unit having an AC side connected to the secondary winding ofone of the array transformers; and mutually independent DC circuitsrespectively connected to a DC side of one of the AC-DC converter units,wherein, by turning on the current bypass device of the primary windingof a specific array transformer and turning off the switches at the endsof the primary winding of the specific array transformer, the specificarray transformer and the AC-DC converter unit connected to the specificarray transformer are isolated.
 2. The power converter according toclaim 1, including a plurality of the AC-DC converter units connected toeach of the secondary windings of each of the array transformers at theAC sides of the AC-DC converter units, wherein the DC sides of theplurality of AC-DC converter units connected to the secondary windingsof each of the array transformers are connected to each of a respectivecommon DC circuit, the common DC circuits connected to the DC sides ofrespective AC-DC convert units being independent of each other.
 3. Thepower converter according to claim 1, wherein each of the arraytransformers includes a plurality of transformers connected in series.4. A power converter, comprising: multiple array transformers havingrespective primary and secondary windings, the primary windings beingconnected in series and to a line; normally-on switches respectivelyconnected in series with corresponding ends of each of the primarywindings of the array transformers; normally-off first current bypassdevices connected in parallel with corresponding primary windings of thearray transformers and the switches connected to the ends of the primarywinding of the corresponding array transformer; AC-DC converter units,each AC-DC converter unit having an AC side respectively connected tothe secondary winding of one of the array transformers; mutuallyindependent DC circuits respectively connected to a DC side of one ofthe AC-DC converter units; and a normally-off second current bypassdevice connected in parallel with all of the series-connected arraytransformers, wherein, by turning on the first current bypass device ofthe primary winding of a specific array transformer and turning off theswitches at the ends of the primary winding of the specific arraytransformer, the specific array transformer and the AC-DC converter unitconnected to the specific array transformer are isolated.
 5. The powerconverter according to claim 4, including a plurality of the AC-DCconverter units connected to each of the secondary windings of each ofthe array transformers at the AC sides of the AC-DC converter units,wherein the DC sides of the plurality of AC-DC converter units connectedto the secondary windings of each of the array transformers areconnected to each of a respective common DC circuit, the common DCcircuits connected to the DC sides of respective AC-DC converter unitsbeing independent of each other.
 6. The power converter according toclaim 4, wherein each of the array transformers includes a plurality oftransformers connected in series.